The invention relates to a device for the welding of plastic films by means of hot gas according to the preamble of claim 1.
Such devices generally serve for the continuous welding of thermoplastic films, i.e., the welding is done with a relative movement between device and film. An intermittent operation is also possible, but the air supply is generally switched off during a standstill.
Such a welding device is described in EP 0 743 164 B1. Although the temperature of the hot gas in the specified device can be regulated well, this applies only to the mean temperature of the hot gas flow as a whole. It has been found that the temperature of the hot gas emerging from an outlet opening situated further away from a gas inlet opening has a higher temperature than the hot gas, which is blown through an outlet opening situated closer to a gas inlet. This may result in irregularities during the welding of plastic film. Thus, the temperature of the hot gas at the outlet opening situated close to the gas inlet is not high enough to successfully weld the plastic film. By contrast, an overly high temperature at the gas outlet opening further away from the gas inlet may result in an uncontrolled melting of the plastic film. In addition, an uneven heating of the hot gas results in an overheating of the heating tube in places, so that it may burn through.
Furthermore, such heating tubes are often shaped asymmetrically, as is shown for example in DE 10 2014 010 184 A1. This causes additional path differences traveled by the gas during the heating process inside the heating tube, which intensifies the effect of the different temperatures of the emerging hot gas.
The problem, which the invention proposes to solve is therefore to provide a device with which the gas can be evenly heated and with which the temperature of the emerging gas can be adjusted precisely.